(Invited) Potential and Activities of III-V/Si Tandem Solar Cells
We have demonstrated 20% efficiency (under 1-sun of AM1.5G) with hetero-epitaxially grown GaAs single-junction solar cells on Si. By employing high quality GaAs films on Si, heteroface GaAs thin films solar cells with an Al0.8Gao.2As window layer were fabricated on Si substrates. The thickness of Si substrates was 200mm. The n-GaAs base layer (carrier concentration of 5x1017 cm-3) with a thickness of 1.3-1.5mm was grown on an Al0.6Ga0.4As/GaAs super lattice (SL) layer (20nm/100nm x 5pairs), In0.1Ga0.9 As/GaAs SLS (10nm/10nm x 5 pairs), and thermal cycle annealed GaAs buffer layer (TCA:9000C x 5 times, 2mm thick) which were grown on Si substrates. The thickness and carrier concentration of p-GaAs emitter layer were 0.5 mm and 2x1018 cm-3, respectively. The total epitaxial layer thickness was about 3.5-5 mm. A Si3N4 layer was used as an anti-reflection film. Ti/Ag was used as electrode metal. The cell area was 1 cm2. The total-area conversion efficiency of 18.3% at AM0 and 20.0% at AM1.5G were the highest ever reported for GaAs-on-Si single-junction solar cells. Such high-efficiency GaAs-on-Si cells have been attained by combining TCA, SLS and SL buffer layers. As a result of such a combination, high efficiency GaAs-on-Si solar cells have been realized using high quality GaAs films with a dislocation density of 3.7x106 cm-2. The first space flight of forty-eight 2 cm x 2 cm GaAs-on-Si solar cells with an average AM0 total-area efficiency of 16.9% have been carried out using the Engineering Test Satellite No. 6 (ETS-VI). The GaAs-on-Si solar cells have been demonstrated to be more radiation-resistant on space than GaAs-on-GaAs solar cells and 50, 100 and 200-mm-thick Si space solar cells. These results show that the III-V/Si-based multi-junction solar cells have great potential for space applications. Ohio State University has also demonstrated 17.1% (AM0) efficiency with GaAs single-junction solar cells by using Ge buffer layer. Nagoya Inst. Tech. has achieved 22.1% with GaAs/Si tandem solar cell by hetero epitaxy. Most recently, Fraunhofer ISE has demonstrated 27.9 % efficiency (under 48.3-suns of AM1.5D) InGaP/GaAs/Si 3-junction solar cells by using epitaxial lift-off and direct bonding.
Future prospects are also discussed. Further improvement in efficiency for GaAs-on-Si single-junction solar cells and III-V/Si multi-junction solar cells can be obtained by reducing dislocation densities to less than 3x105 cm-2.